1. Field of the Invention
This invention relates to methods and compositions for the hydraulic fracturing of subterranean formations. It more particularly relates to methods and compositions for fracturing a subterranean formation penetrated by a well bore wherein a fluid composition having retarded crosslinking properties is injected into a formation through a suitable conduit at a rate and pressure sufficient to produce a fracture in the formation.
2. Brief Description of the Prior Art
In the drilling, completion and treatment of subterranean formations penetrated by well bores, viscous treating fluids commonly are utilized. In such operations, it often is desirable or necessary that the viscous treating fluids have relatively low initial viscosities, but when placed in the well bore or subterranean formation to be treated, the viscosities of the fluids increase. For example, in performing a subterranean fracturing process on a hydrocarbon-bearing formation to stimulate the production of hydrocarbons therefrom, a treating fluid which has a low viscosity and a low friction pressure when being pumped but which exhibits a high viscosity in the formation is desirable.
Generally, in the art of hydraulic fracturing, a fluid is introduced through a conduit, such as tubing or casing, disposed in the well bore into a formation sought to be fractured. The fluid is introduced at a rate and pressure sufficient to produce a fracture or fractures in the formation and to extend the produced fracture or fractures from the well bore into the formation. Upon the creation of the fracture or fractures, additional fracturing fluid containing solid proppant materials can be introduced into the fracture or fractures in the event the initial fluid did not contain any proppant. Following this treatment, the introduced fluid is recovered from the formation, but the proppant remains in the produced fracture or fractures to thereby prevent the complete closure thereof. The propped fracture creates a conductive channel extending from the well bore into the formation.
The conductivity of a propped fracture is effected by the particle size of the proppant material placed in the fracture. The particle size of the proppant that can be used depends upon the width to which the particular fracture can be opened during the introduction of the fracturing fluid. The fracture width normally is directly proportional to the viscosity of the fracturing fluid. In addition, the use of fracturing fluids having relatively high viscosities is advantageous since such fluids can support the proppant particles suspended therein without excessive settling. The use of such high viscosity fluids also permits the placement of relatively large-size proppant material in the fracture without a screenout occurring, that is, without the proppant bridging across the mouth of the fracture and preventing the introduction of proppant therein.
The use of desirably high viscosity fracturing fluids undesirably is accompanied by the problem of high friction losses usually encountered during the introduction of such fluids into a formation through the conduit, such as tubing or casing, disposed in the well bore. Since the pumping equipment and tubular goods are limited in capacity and operating pressure, the viscosity of the fluid being pumped also is limited. The viscosity of the fluid must be low enough that excessive friction losses and high well head pumping pressures are not encountered.